Pivoting or oscillating torsional hinged mirrors provide very effective yet relatively inexpensive replacements for spinning polygon shaped mirrors in printers and some types of displays. As will be appreciated by those skilled in the art, torsional hinged mirrors may be MEMS type mirrors etched from a silicon substrate or wafer using processes similar to those used in the manufacture of semiconductor devices. Some versions of torsional hinge mirrors for providing a raster type scan for printers and displays operate at rotational speeds no greater than about 3 KHz or less, and can be manufactured thick enough so that they avoid serious flatness problems with respect to the reflective surface. However, as the demand for higher print speeds and better resolution increases, flatness of the mirror reflective surface becomes a much more serious problem. As the mirror continuously flexes or bends back and forth during the continuous oscillations about the axis, the greatest deformation is at the tip or ends of the flexing mirror. This problem has been partially eliminated by the use of center spines that extend along the long axis of the elliptical shaped mirror to each of the tips or ends of the mirror. In addition, smaller and even thinner mirrors with even greater rotational speeds are also affected by flexing modes around the mirror edges during high speed operations.
Unfortunately a hinge plate alone with center spines simply is not effective to assure the necessary flatness or to sufficiently reduce the various types of flexing. Of course, the oscillating member, the hinge plate or mirror itself can be made thicker, but this increases the weight and mass to an unsatisfactory level. Consequently, in addition to a hinge plate with center spines, the mirror layer may now be made from a small thicker layer of silicon that is formed or etched at two levels. The front level shape is the thin elongated mirror and the second level shape is a supporting truss structure that also includes center spines. However, even including an etched truss structure does not maximize flatness of the mirror.
Earlier versions of the etched mirror plate or oscillating member simply included center spines that were matched with and bonded to the center spines of the hinge plate. However, some of the high speed mirrors that also flex around the edges of the mirror require more complex truss structures that include ridge members located along selected portions of the mirror edges. These complex truss structures can be etched in the hinge plate alone or in both the hinge plate and the oscillating member.
As will be appreciated by those skilled in the art, the use of such complex truss structures slow the speed of manufacturing and increases the cost. Referring now to FIGS. 8A, 8B, 8C, and 8D, there is shown as an example only, a torsional hinged device manufactured by presently available etching processes. FIG. 8A illustrates an assembled torsional hinged device, which includes a hinge plate 10, a multilevel structure, which in the illustrated example is an oscillating mirror structure 12 bonded to a front side of the hinge plate 10 and a permanent magnet 14 bonded to the back side of the hinge plate 10. FIG. 8B is the hinge plate 10 alone and FIGS. 8C and 8D are two perspective views of the oscillating mirror structure or mirror plate alone. More specifically, and referring to FIG. 8B, the hinge plate 10 includes a pair of support anchors 16a and 16b which will be mounted to a support structure (not shown), a pair of torsional hinges 18a and 18b extending from the support anchors 16a and 16b to a center support member 20 having a front side 22a and a back side 22b. Also included on the central support member 20 are first and second center spines 24a and 24b. The hinge plate 10 was typically formed in the prior art by a single etch process that required a single mask for the single perimeter etch. That is, a layer or wafer silicon of the desired thickness is etched completely through in a single etch process. To assure that the hinges can withstand the stress generated during rotation of the mirror, the single etch process is optimized to form substantially straight and very smooth side walls.
On the other hand, to form the oscillating structure or mirror part of FIGS. 8C and 8D according to the prior art is a two step etching process. For example, there is a first mask and a first etch process that forms the truss structure 26 with spines 28a and 28b as a first level shape. After the first level shape or truss structure 26 with spines 28a and 28b etched in the back side of a silicon layer, a second mask and a second etch process is used to etch completely through the silicon wafer or layer to form the second level or perimeter shape 30 of the oscillating structure or mirror part 12. Since the oscillating structure or mirror part 12 requires two different etch steps, the etching process is optimized for speed. Unfortunately, such fast etching results in undercutting the truss structure such as is clearly shown on the spine 28b by arrows 32a and 32b. As will be appreciated by those skilled in the art, the undercutting results in less mass or material in the area where the first level or truss structure joins the back side of the second level or mirror shaped portion than is present at the very back side 40 of the truss structure. This is exactly opposite of a preferred truss structure, which would include a larger contact area between the two level shapes and a reduced area at the very back surface of the truss structure.
Therefore, it would be advantageous to provide a faster and less expensive method of manufacturing a mirror plate or other structure for use with a torsional hinged device that includes a truss structure having superior flatness and that reduces flexing at the mirror edges.